Atom line intensity

The ICP is almost in local thermal equilibrium. Indeed, the excitation temperatures (from atomic line intensity ratios) are about 6000 K [382] and the rotation temperatures (from the rotation lines in the OH bands) are 4000-6000 K (see Refs. [383, 384]). From the broadening of the Hg-line, an electron number density of 1016 cm 3 is obtained, whereas from the intensity ratio of an ion and an atom line of the same element the electron number density found is 1014 cm-3. It has also been reported that measured line intensity ratios of ion to atom lines are higher by a factor of 100 than those calculated for a temperature of 6000 K and the electron number density found is 1016 cm 3. This indicates the existence of over-ionization. This can be understood from the excitation processes taking place. They include the following. 

As in inductively coupled plasma optical emission (ICP-OES) spectra, in addition to atomic lines, intense ionic lines are also observed, the use of an ICP as an ion source for MS seemed logical, but overcoming the difference in pressure between the ICP (generated at atmospheric pressure) and the mass spectrometer (10 —10 mbar) proved difficult and had to be accomplished via the use of a two-cone interface. Despite the advantages that double-focusing sector field mass spectrometers (higher mass resolution) and TOP analyzers (high data acquisition speed) can offer, approximately 90% of the ICP-MS units used worldwide are equipped with a quadrupole filter for mass analysis. 

The spatial localization of H atoms in H2 and HD crystals found from analysis of the hyperfine structure of the EPR spectrum, is caused by the interaction of the uncoupled electron with the matrix protons [Miyazaki 1991 Miyazaki et al. 1991]. The mean distance between an H atom and protons of the nearest molecules was inferred from the ratio of line intensities for the allowed (without change in the nuclear spin projections. Am = 0) and forbidden (Am = 1) transitions. It equals 3.6-4.0 A and 2.3 A for the H2 and HD crystals respectively. It follows from comparison of these distances with the parameters of the hep lattice of H2 that the H atoms in the H2 crystal replace the molecules in the lattice nodes, while in the HD crystal they occupy the octahedral positions. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

The intrinsic drawback of LIBS is a short duration (less than a few hundreds microseconds) and strongly non-stationary conditions of a laser plume. Much higher sensitivity has been realized by transport of the ablated material into secondary atomic reservoirs such as a microwave-induced plasma (MIP) or an inductively coupled plasma (ICP). Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms compared with at most a hundred microseconds in a laser plume) and because of additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of these factors the DLs of LA-MIP have been improved by one to two orders of magnitude compared with LIBS. 

As indicated in Fig. 21.3, for both atomic absorption spectroscopy and atomic fluorescence spectroscopy a resonance line source is required, and the most important of these is the hollow cathode lamp which is shown diagrammatically in Fig. 21.8. For any given determination the hollow cathode lamp used has an emitting cathode of the same element as that being studied in the flame. The cathode is in the form of a cylinder, and the electrodes are enclosed in a borosilicate or quartz envelope which contains an inert gas (neon or argon) at a pressure of approximately 5 torr. The application of a high potential across the electrodes causes a discharge which creates ions of the noble gas. These ions are accelerated to the cathode and, on collision, excite the cathode element to emission. Multi-element lamps are available in which the cathodes are made from alloys, but in these lamps the resonance line intensities of individual elements are somewhat reduced. 

Choi and Funayama [19] also measured sodium atom emission from sodium dodecylsulfate (SDS) solutions in the concentration range of 0.1-100 mM at frequencies of 108 kHz and 1.0 MHz. The sodium line intensity observed at 1 MHz was nearly constant in the concentration range from 3 to 100 mM and was considerably higher than that at 108 kHz. This frequency dependence of the intensity is opposite that for NaCl aqueous solution. The dynamical behavior of the absorption and desorption of surfactant molecules onto the bubble surface may affect the reduction and excitation processes of sodium atom emission. This point should be clarified in the future. 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

The XANES region of the Pt Lm and Ln absorption edges can be used to determine the fractional d-electron occupancy of the Pt atoms in the catalyst sample by a so-called white line analysis. Figure 2 shows the XAS spectrum collected at both Pt Lm and Lii absorption edges of Na2Pt(OH)e. The sharp features at the absorption edges are called white lines after the white line observed in early photographic film based XAS measurements. Mansour and coworkers have shown that comparison of the white line intensities of a sample with those of a reference metal foil provides a measure of the fractional d-electron vacancy, f, of the absorber atoms in the sample. is defined as follows ... 

Figure 20. Structural parameters as a function of time extracted by fitting the data shown in Figure 20. (A) Data collected during the oxidation of the Pt/C electrode and (B) during the reduction long dashes, first shell O coordination number (no. of O atoms) short dashes, first shell Pt coordination number (no. of Pt atoms) solid line, absorption peak intensity (effectively white line intensity).(Reproduced with permission from ref 43. Copyright 1995 Elsevier Sequoia S.A., Lausanne.)...

Spectral lines are often characterized by their wavelength and intensity. The line intensity is a source-dependent quantity, but it is related to an atomic constant, the transition probability or oscillator strength. Transition probabilities are known much less accurately than wavelengths. This imbalance is mainly due to the complexity of both theoretical and experimental approaches to determine transition probability data. Detailed descriptions of the spectra of the halogens have been made by Radziemski and Kaufman [5] for Cl I, by Tech [3] for BrIwA by Minnhagen [6] for II. However, the existing data on /-values for those atomic systems are extremely sparse. 

The NMR spectra of Pbs and Sn5 have not been reported even though the clusters were first isolated in 1975 [40]. Preliminary data for Pb5 ion show that it is static on the NMR time scale (Fig. 4) [41]. The Dsh structure of the trigonal bipyramidal cluster is expected to give rise to two NMR resonances. While only one resonance has been located (5 ° Pb = -3,591 ppm), it clearly shows coupling to a second inequivalent set of Pb atoms with j( Pb- ° Pb = 2,344 Hz). The calculated intensity distribution for the three equatorial Pb nuclei is expected to be 0.004 0.28 1 0.28 0.004, whereas the signal for the two axial Pb atoms is expected to be 0.054 0.387 1 0.387 0.054. Although it is difficult to compare the observed line intensities for the assignment, experimental data are closer to the... 

A comparison of the X-ray absorption edges of bulk gold and Aujs [39,40] indicates that all the main features of the edge of the bulk metal are also exhibited by the cluster, the white line intensity showing qualitatively that the gold atoms in the cluster have a low mean oxidation state [150], closely approaching that in the bulk. 

Here I stands for the intensity of the spectral hnes N is the atom number density in cm Z is the partition function E and y are the energies and degeneracy s of the upper levels, respectively and A and A are the Einstein coefficient and wavelength, respectively, for the observed transitions. When changing the concentration Nt relative to that Nm the line intensities ft and 7m will likewise change, and according to (6.1) one should obtain a cahbration curve with constant slope (Davies et al. 1995 Ciucci et al. 1999 Hou and Jones 2000). 

Thus, spectral interferences in atomic spectroscopy are less likely than in molecular spectroscopy analysis. In any case, even the atomic lines are not completely monochromatic i.e. only one wavelength per transition). In fact, there are several phenomena which also bring about a certain broadening . Therefore, any atomic line shows a profile (distribution of intensities) as a function of wavelength (or frequency). The analytical selectivity is conditioned by the overall broadening of the lines (particularly the form of the wings of such atomic lines). 

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